In integrated circuit technology, there is virtually an unlimited number of instances where a voltage level must be analyzed in order to determine whether it falls above or below a particular threshold. The specific value of the threshold voltage is dependent upon the overall circuit or system which provides the analyzed voltage.
The prior art has provided various techniques for detecting threshold voltages. Many of these techniques have both advantages and disadvantages associated with them. One known technique includes a circuit having an input stage wherein two P channel transistors are connected to an N channel transistor. In this technique, the P channel transistors must be constructed in one area on the semiconductor device while the N channel transistor must be constructed in a separate area. For example, the N channel transistor may be formed on a P type semiconductor substrate, while the P channel transistors may be formed in a separate N tank formed within the P type substrate. Thus, a considerable amount of area is consumed in order to lay out this prior art circuit.
An additional disadvantage with the above-described prior art technique arises from the increased possibility of a latch-up condition. The combination of P channel and N channel transistors and the possibility of high input voltages increase the possibility of this latch-up condition. Latch-up occurs where the so-called parasitic devices within the circuit inadvertently are turned on and remain on. Parasitic devices are undesirable inherent effects that naturally occur due to certain characteristics of the circuit. Once the parasitic devices turn on, the circuit most likely will not work properly. Indeed in many instances, once a latch-up condition has occurred, this condition may remain even though the inputs to the circuit are changed. As a result, the circuit may not only work improperly, but in addition, damage may occur to both the circuit and the system containing the circuit. Thus, it may be appreciated that this prior art input configuration is often highly undesirable.
Therefore, a need has arisen for a threshold detection circuit which minimizes the amount of surface layout necessary for the circuit, isolates the relative high voltages of the circuit and substantially reduces the possibility of the circuit going into a latch-up condition.